The term vitamin is derived from the words vital and amine, because vitamins are required for life and were originally thought to be amines. Although not all vitamins are amines, they are organic compounds required by humans in small amounts from the diet. An organic compound is considered a vitamin if a lack of that compound in the diet results in overt symptoms of deficiency.

Vitamins are essential, meaning that our body can not produce them itself. We need to supply them daily in our diet or through supplements. Each vitamin fulfills its unique function in the metabolism.

You need only small amounts (that's why they are often referred to as micronutrients) because the body uses them without breaking them down, as happens to carbohydrates and other macronutrients. So far, 13 compounds have been classified as vitamins. Vitamins A, D, E, and K, the four fat-soluble vitamins, tend to accumulate in the body. Vitamin C and the eight B vitamins-biotin, folate, niacin, pantothenic acid, riboflavin, thiamin, vitamin B6, and vitamin B12-dissolve in water, so excess amounts are excreted.

The "letter" vitamins sometimes go by different names. These include:

  • Vitamin A = retinol, retinaldehyde, retinoic acid
  • Vitamin B1 = thiamin
  • Vitamin B2 = riboflavin
  • Vitamin B3 = niacin
  • Vitamin B5 = pantothenic acid
  • Vitamin B6 = pyridoxine, pyridoxal, pyridoxamine
  • Vitamin B9 = folic acid
  • Vitamin B12 = cobalamin
  • Vitamin C = ascorbic acid
  • Vitamin D = calciferol
  • Vitamin E = tocopherol, tocotrienol
  • Vitamin H = biotin
  • Vitamin K = phylloquinone